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In the last decade, we have witnessed the wide adoption of a variety of wireless technologies like WiFi, Cellular, Bluetooth, ZigBee, and Near-field Communication(NFC). However, the fast growth of wireless networks generates significant cross-technology interference, which leads to network performance degradation and potential security breach. In this dissertation, we propose two novel physical layer techniques to deal with the interference, and improve the performance and security of sensor... Show moreIn the last decade, we have witnessed the wide adoption of a variety of wireless technologies like WiFi, Cellular, Bluetooth, ZigBee, and Near-field Communication(NFC). However, the fast growth of wireless networks generates significant cross-technology interference, which leads to network performance degradation and potential security breach. In this dissertation, we propose two novel physical layer techniques to deal with the interference, and improve the performance and security of sensor networks and mobile systems, respectively. First, we exploit the WiFi interference as a ``blessing" in the design of sensor networks and develop novel WiFi interference detection techniques for ZigBee sensors. Second, utilizing these techniques, we design three efficient network services: WiFi discovery which detects the existence of nearby WiFi networks using ZigBee sensors, WiFi performance monitoring which measures and tracks performance of WiFi networks using a ZigBee sensor network, and time synchronization which provides synchronized clocks for sensor networks based on WiFi signals. Third, we design a novel, noninvasive NFC security system called {\em nShield} to reduce the transmission power of NFC radios, which protects NFC against passive eavesdropping. nShield implements a novel adaptive RF attenuation scheme, in which the extra RF energy of NFC transmissions is determined and absorbed by nShield. At the same time, nShield scavenges the extra RF energy to sustain the perpetual operation. Together with the extremely lo-power design, it enables nShield to provide the host uninterrupted protection against malicious eavesdropping. The above systems are implemented and extensively evaluated on a testbed of sensor networks and smartphones. Show less